Optical waveguide devices require a high degree of accuracy in the dimensions of the optical waveguiding layer. In the past it has not been possible to achieve the degree of parallelism and uniformity in a wafer scale manufacturing process that would enable an adequate yield across the wafer, and from wafer to wafer. A wafer of optically transmissive material thinned and polished to waveguide dimensions of approximately 1-10 microns does not have the dimensional stability to be polished to or to hold the flatness required, or even to be handled without breaking. In order to achieve the degree of parallelism required in a thin optical material, a master reference plane must be provided in a carrier substrate wafer. It is known in the art to polish wafers to a uniformity of parallelism within 0.005 microns. However, for wafer scale manufacture, this precision is not transferred to the thin waveguiding layer due to variation introduced by the adhesive layer securing the waveguiding layer to the carrier substrate. Optical devices such as planar lightwave circuits, electro-optic modulators and ridge waveguide devices are examples of optical waveguide devices for which dimensional uniformity is essential to achieve wafer scale production.
Second harmonic generation applications (frequency doubling) using ridge waveguide devices have been developed to create laser output in the ultraviolet, visible and infrared wavelength spectrum for use in numerous technologies. Demand for these devices is high. However, the manufacture of ridge waveguide devices has been limited to individual device processing. One problem that arises in the manufacture of ridge waveguides for the application of second harmonic generation is the control of dimensions of the ridge. In particular the thicknesses of the ridge waveguide must be controlled accurately, due to sensitivity of the upconversion wavelength to all dimensions of the ridge. Lateral dimensions are controlled with photolithographic processes, while vertical dimensions are controlled by etching and polishing processes.
Ridge waveguide devices for second harmonic generation considered in this application have periodically poled regions in the waveguides for phase matching the pump and output signals. Adhesive assembly of the supporting carrier wafer and optically transmissive wafer is preferred. In order to achieve adequate manufacturing yield, the range of thickness of the ridge and planar slab region must be controlled to within a few tenths of a micron. In order to achieve this level of uniformity across the wafer and from wafer-to-wafer, the thickness of the adhesive between the transmissive wafer and carrier wafer must be controlled to within this same range. This level of control for wafer scale manufacture has not been demonstrated in the prior art.
An optical wavelength conversion element is disclosed in U.S. Pat. No. 6,631,231 by Kiminori Mizuuchi et al. issued to Matsushita Electric Industrial Co. Ltd. on Oct. 7, 2003. In this patent a continuous joining layer of amorphous material is used to join a poled waveguide structure to a substrate layer. No method for controlling the adhesive thickness is disclosed, although the disclosure does recognize some critical optical limitations to adhesive thickness. Instead, significant post assembly finishing is disclosed. These are labor intensive methods for individual device production. Such methods do not produce high yield. Furthermore, the Mizuuchi design is dependent on the optical properties of the adhesive layer, which limits the design choice.
Adhesive layer spacing is known in various optical industries. For instance glass fiber particles are disclosed for use as spacers between LCD display screen layers in U.S. Pat. No. 4,390,245. U.S. Pat. No. 6,896,949 disclosing the wafer scale manufacture of etalons also makes use of small beads to facilitate spacing between plural assembled etalons, or of fritted glass which is applied to a certain thickness and heated to its melting point to join etalon elements. An image sensor as disclosed in U.S. Pat. No. 5,433,911 also discloses the assembly of an individual device, using spacers constructed through resist patterning in order to secure a protective cover with controlled parallelism. However, none of these disclosures provide instruction for adhesive assembly suitable for the present application that can provide the level of accuracy necessary for wafer scale production control across the wafer and from wafer to wafer. Beads and spacers available in the industry do not provide the uniformity within 0.1 microns deviation needed to ensure tolerance control across the wafer.
A wafer scale manufacturing process for optical waveguide devices, and ridge waveguide devices in particular, remains highly desired in the industry.
An object of the present invention is to provide a wafer scale manufacturing process for producing optical waveguide devices with sub micron accuracy and high yield.
It is a further objective of the present invention to provide a ridge waveguide device made in accordance with the manufacturing process of the present invention including rigid spacing elements within a precisely dimensioned discontinuous adhesive layer.